Toward a non-invasive cleaning of the wall painting using polyelectrolyte hydrogel

Developing advanced cleaning systems for the removal of unwanted layers from painted surfaces is still a challenging issue that needs to be addressed in the field of cultural heritage. In this work, we proposed the use of novel polyelectrolyte hydrogels prepared via free radical polymerization and freeze-thaw methods. Zwitterionic sulfobetaine methacrylate (SBMA) was cross-linked with methylacryloyloxyethyl trimethylammonium (DMC) through covalent bonding to yield zwitterionic polyelectrolyte poly(SBMA-co-DMC) (PSD). Introducing PSD into the polyvinyl alcohol/carboxymethyl chitosan (PVA/CMC) hydrogel network enhanced the intermolecular electrostatic interactions and changed the porous structure and swelling behaviors, thus leading to improved mechanical properties. The adsorption ability of the PVA/CMC/PSD polyelectrolyte hydrogels toward dyes (rhodamine B and congo red) and metal ions (Cu2+ and K+) varied with increasing amounts of PSD, which could be attributed to cationic PSD tuning the net charges in the hydrogel. The cleaning effectiveness of the PVA/CMC/PSD polyelectrolyte hydrogels was assessed on wall painting mock-ups coated with soil, red paint, and soluble salts, using microscopic and spectroscopic analyses. The results showed that the hydrogel adhered to rough surfaces and removed unwanted substances effectively without leaving determinable residues, providing a possible safe and efficient method for the cleaning of important artifacts.